Recent advances in the transmission of luminous signals through light-conducting fibers or ribbons have made their use attractive for telecommunication purposes. Thus, signal paths constituted by such light conductors can have lengths on the order of tens of kilometers between repeaters, thanks to their low attenuation constants.
Since such light conductors are usually produced only in sections of limited length, long-distance transmission lines often must be spliced together from a large number of such sections. The splicing of the fibers or ribbons is a delicate and therefore time-consuming operation; aside from the cost involved, every splice also entails a loss of energy due to unavoidable differences in the diameters of the conductor cores and/or their sheaths as well as in the optical characteristics of the vitreous core material.
Some methods, such as the double-crucible technique or the Verneuil process, have already been proposed for the continuous manufacture of optical-fiber trunks designed to obviate the need for splices between repeater stations. These prior methods, however, have limited applicability especially in cases in which the refractive index of the fiber is to vary according to a predetermined law. Thus, conventional procedures allow this index to be modified only in a discontinuous manner. There is also the problem of avoiding contamination of the core material.